Gas is one of the primary sources of energy used for both domestic as well as industrial uses. Natural gas, usually comprised of a mixture of hydrocarbons, is transported through transportation pipelines over long distances and then distributed in distribution networks or grids.
Gas is transported in the transportation pipelines at relatively high pressure, for instance in the range of 55-65 bar. In a distribution network, where through gas is distributed to final users, gas is present at a substantially lower pressure, e.g. in the range of 6-24 bar, depending upon local legislation. Pressure-let-down stations are used, wherein gas flows from the transportation pipeline towards the distribution network while the pressure of the gas is reduced as required.
In commonly used pressure-let-down stations the gas is caused to expand in pressure control valves, also called Joule Thomson (JT) valves. The entire energy associated to the pressure is dissipated in the pressure control valve. During decompression the gas cools down due to the Joule-Thomson effect. The heavier hydrocarbons present in the gas can condensate if too a low gas temperature is achieved at the end of the decompression process. In order to prevent condensation and/or formation of hydrates, a minimum admissible temperature at the inlet of the distribution network is usually set by legislation. The minimum temperature is usually around 0-5° C. Thus, before decompression, the gas is heated such that the final temperature thereof, after decompression, does not drop below the minimum admissible gas temperature at the inlet of the distribution network.
Compressed gas is usually heated by heat exchange against hot water, which is in turn produced in a boiler, where a portion of the gas transported in the distribution network is burned to generate heat. Depressurizing gas from a first, transportation pressure to a second, distribution pressure, lower than the transportation pressure, is thus an energy-consuming process due to two factors: on the one hand the pressure energy present in the gas is dissipated. On the other hand, a certain amount of gas must be consumed just for the purpose of heating the high-pressure gas to prevent the temperature of the low-pressure gas to drop below the minimum admissible gas temperature.
Attempts have been made to make the depressurization process less energy-consuming, by recovering the pressure energy from the gas. For this purpose, gas is expanded in a turboexpander, e.g. a radial turbine, which converts at least part of the pressure energy contained in the gas flow into mechanical power. The latter can then be exploited as such or converted into electric power by means of an electric generator.
However, the pressure drop being the same, an expansion process through a turboexpander, which generates mechanical power, causes a much higher temperature drop than a JT valve. This is simply corresponds to the fact that the gas transformation is not an adiabatic transformation, but becomes a quasi-isentropic transformation, during which power is extracted from the flow of expanding gas.
In order to meet the temperature requirements at the inlet of the gas distribution grid, therefore, more thermal power must be spent in order to heat the high-pressure gas at a temperature higher than that required if a simple JT valve is used for depressurizing purposes. Considering the revenue stream generated by the power generated by the expander versus the extra-amount of expenditure for heating, the resulting margin is so slim that it rarely justifies the higher investments required by complex machinery, as the turboexpander and electric generator.
U.S. Pat. No. 8,028,535 suggests using a transcritical heat pump as a source of heat for heating the gas in a gas depressurization system. The use of a transcritical heat pump can, under certain operating conditions, result in a more efficient depressurization system, in view of the high coefficient of performance which a transcritical heat pump can achieve when operating between the ambient temperature and the high temperature required to be achieved by the gas to be expanded in the depressurization system.
However, it turned out that depressurization systems using transcritical heat pumps may not be expedient under certain operating conditions, namely when the gas flow rate is reduced with respect to a design flow rate through the expander. It shall be noted that, due to the kind of use made of this source of energy, the gas flow rate has dramatic daily fluctuations, e.g. since at nighttime a much smaller amount of gas is required. Strong yearly fluctuations are also to be taken into account, due to larger consumption of gas during the cold season, as well as due to variations of gas consumption linked to variations in the industrial activity, which also can vary during the year.
Similar limitations and drawbacks are also encountered if a standard, i.e. non-transcritical, heat pump is used as a heating means for heating the pressurized gas prior to expansion in the expander.
A need therefore exist, for further improving the efficiency of pressure-let-down stations using expanders and heat pumps as sources of heat to increase the gas temperature.